Millions of tons of COPR were historically generated in conjunction with the extraction of chromium from chromium-bearing ores. Containing high concentrations of chromium in trivalent and hexavalent valence states, COPR deposits leach chromium into the environment over very long time frames. The mechanisms controlling this leaching are complex and not well understood. With an intricate mineralogy, highly alkaline pH, and an aggressive ion exchange capacity, COPR represents a very challenging treatment problem. Thus, the cost-effective treatment of COPR waste has proven difficult.
The leaching of soluble chromium from most chromium-bearing wastes can be controlled through the application of any number of chemical reducing agents. These treatment reagents reduce hexavalent chromium, which is soluble and exhibits a high toxicity, to the less toxic and less soluble trivalent state.
However, these approaches have not proven effective for high concentration COPR deposits. While COPR has been successfully treated at Resource Conservation and Recovery Act (hereinafter sometimes RCRA)-permitted facilities for many years, most of this treated material has consisted of COPR contaminated soils with total chromium concentrations from 200 to 4,000 mg/kg. In these relatively low concentration materials, 1% to 8% of the total chromium is typically present as hexavalent chromium (hereinafter sometimes Cr(VI)). At these relatively low total chromium concentrations, conventional chromium treatment technologies can reduce the toxic characteristic leaching procedures (hereinafter sometimes TCLP) chromium to below the Universal Treatment Standard (hereinafter sometimes UTS) of 0.60 mg/kg, and remain stable.
For sites highly enriched with COPR, the total chromium concentration can exceed 27,000 mg/kg, with 30% to 60% existing as Cr(VI). Research shows that conventional treatment reagents can often achieve the UTS for chromium when applied to these concentrated COPR wastes, but within weeks chromium begins to remobilize and resume leaching at high concentrations.
One study performed by RMT, Inc., 744 Heartland Trail (53717-1934), P.O. Box 8923 (53708-8923), Madison, Wis., illustrates this pattern. An alkaline COPR waste was treated with 10% by weight of ferrous sulfate heptahydrate and 5% by weight ferric sulfate. The treated material was then screened for TCLP chromium over a period of four weeks with the following results:
Time Elapsed After TreatmentTCLP Cr Concentration (mg/l)0 days0.83 days2.67 days7.028 days 14.2
The issue of remobilization/re-oxidation of the chromium is rarely (if ever) confirmed at RCRA-permitted facilities. By permit, these facility operators treat the COPR, test it post-treatment, and then landfill the material at a RCRA Subtitle D or RCRA Subtitle C cell unaware that the treated COPR will soon be contributing hexavalent chromium to their landfill leachate. It is postulated that any treatment of COPR with total chromium concentrations exceeding 10,000 mg/kg needs to be tested weeks after treatment to confirm the long-term stability of any treatment technology employed. Failure to conduct such testing could result in the creation of unwanted long-term liabilities.
Enriched COPR wastes display a slow dissolving mineral-based alkalinity that is difficult to overcome without the addition of significant quantities of strong acids. Treatment approaches that can permanently reduce the pH of concentrated COPR wastes may avoid this Cr remobilization pitfall. Although these methodologies achieve the desired TCLP limits, they generally have not been shown to be cost-effective. Also, the costs and dangers associated with working with large quantities of strong acids makes these approaches virtually impractical to implement on a large scale.
Other effective treatment regimens for COPR waste are based upon the usage of various reagents. Although effective, the high reagent concentrations required for successful treatment cause large-scale implementation to be cost-prohibitive. Also, for large COPR clean-up projects, the demand for traditional reagents most likely will exceed the available supply of the required reagents. The ability of COPR waste to “consume” large reagent volumes appears to derive from the aggressive ion exchange capacity of COPR. Research on COPR waste conducted by Geelhoed, et al. demonstrated that hydrocalumite, one of the chromium-bearing mineral phases present in COPR, readily undergoes anion exchange. Their research showed that treatment with ferrous sulfate, a commonly used reagent for columbium reduction, actually results in increased leaching of chromium from COPR. This occurred when the sulfate anion exchanges for the chromate anion in the hydrocalumite.
Gancy et al U.S. Pat. No. 3,981,965 (hereinafter sometimes Gancy) teaches that one group of compounds that perform quite satisfactorily in an alkaline medium to reduce soluble chromium compounds to the insoluble form, are those organic compounds which have labile sulfide atoms and therefore behave as slow-release sulfide reagents. Among this group of organic compounds are thiourea, thioglycolic acid, sodium xanthate, thioacetamide, and bis(dimethylthiocarbamoyl)disulfide.
Gancy further teaches that of the two preferred sulfides, namely sodium hydrosulfide and calcium sulfide, each appears to behave in a unique manner. The sodium hydrosulfide is quite soluble and could be expected to form polysulfides as does the more alkaline sodium sulfide. Gancy explains that the yellow color of the polysulfide generally in evidence when sodium hydrosulfide is first used as a reductant, but this color is transient, and soon disappears. The objection to the yellow color of the polysulfides, according to Gancy, stems from its resemblance to the toxic yellow chromate bleed, and although almost indistinguishable in appearance, the polysulfide is substantially non-toxic in comparison with the soluble chromium bleed which it resembles. Gancy thus teaches away from the use of polysulfide in COPR remediation and, in those cases where polysulfide may be formed, recommends suppression of polysulfide formation by the addition of a minor amount of sodium sulfite to the chromite ore processing residue.
A more in-depth analysis of the environmental hazards posed by COPR and various remediation methods are found in the thesis of James Martin Tinjum, “Remediation of Chromium Ore Processing Residue (COPR) and Mitigation of the Impacts on Transportation Facilities,” Apr. 30, 2004, submitted in partial fulfillment of the requirements for a Ph. D. degree in the Department of Civil and Environmental Engineering, University of Wisconsin, Madison, Wis.
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